Method for Producing an Ultra High Strength Coated or Not Coated Steel Sheet and Obtained Sheet

ABSTRACT

A method for producing a cold rolled steel sheet having a tensile strength≧1470 MPa and a total elongation TE≧19%, the method comprising the steps of annealing at an annealing temperature AT≧Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≦C≦0.40%, 1.50%≦Mn≦2.30%, 1.50≦Si≦2.40%, 0%≦Cr≦0.7%, 0%≦Mo≦0.3%, 0.01%≦Al≦0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT&lt;Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by re-heating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 250 seconds.

The present invention concerns the manufacture of coated or non-coatedhigh strength steel sheet having improved tensile strength and improvedtotal elongation and the sheets obtained by this method.

To manufacture various equipment such as parts of body structuralmembers and body panels for automotive vehicles, it is now usual to usebare, electro-galvanized, galvanized or galvannealed sheets made of DP(dual phase) steels multi-phase, complex phase or martensitic steels.

For example, a high strength multi-phase may include abainite-martensitic structure with/without some retained austenite andcontains about 0.2% of C, about 2% of Mn, about 1.5% of Si which wouldresult in yield strength of about 750 MPa, a tensile strength of about980 MPa, a total elongation of about 10%. These sheets are produced oncontinuous annealing line by quenching from an annealing temperaturehigher than Ac3 transformation point, down to an overaging temperatureabove Ms Transformation point and maintaining the sheet at thetemperature for a given time. Optionally, the sheet is galvanized orgalvannealed.

To reduce the weight of the automotive parts in order to improve theirfuel efficiency in view of the global environmental conservation it isdesirable to have sheets having improved strength-ductility balance. Butsuch sheets must also have a good formability.

In this respect, it was proposed to produce sheets made of steel usingso called quenched and partitioned having improved mechanical propertiesand good formability. Coated or non-coated (bare) sheets having, atensile strength TS of about 1470 MPa and a total elongation of at least19%, are targeted. These properties are targeted at least when the sheetis not coated or galvanized.

When the sheet is galvannealed, a tensile strength TS of at least 1470MPa and a total elongation of at least 15%, preferably at least 16% aretargeted.

Therefore, the purpose of the present invention is to provide such sheetand a method to produce it.

For this purpose, the invention relates to a method for producing a coldrolled steel sheet having a tensile strength TS of at least 1470 MPa anda total elongation TE of at least 16%, the method comprising thesuccessive steps of:

-   -   annealing at an annealing temperature AT a cold rolled steel        sheet made of steel whose chemical composition contains in        weight %:

0.34%≦C≦0.40%

1.50%≦Mn≦2.30%

1.50≦Si≦2.40%

0%<Cr≦0.7%

0%≦Mo≦0.3%

0.01%≦Al≦0.08%

and optionally 0%≦Nb≦0.05%,

the remainder being Fe and unavoidable impurities, the annealingtemperature AT being equal or higher than the Ac3 transformation pointof the steel, to obtain an annealed steel sheet,

-   -   quenching the annealed steel sheet by cooling it down to a        quenching temperature QT lower than the Ms transformation point        of the steel, typically between 150° C. and 250° C., to obtain a        quenched steel sheet, and,    -   making a partitioning treatment by reheating the quenched steel        sheet at a partitioning temperature PT between 350° C. and        450° C. and maintaining the steel sheet at this temperature        during a partitioning time Pt between 15 seconds and 250        seconds.

Preferably, during quenching, the annealed steel sheet is cooled down tosaid quenching temperature at a cooling rate fast enough to avoidferrite formation upon cooling, in order to obtain a quenched sheethaving a structure consisting of martensite and austenite.

Preferably, the annealing temperature AT is between 870° C. and 930° C.

According to an embodiment, the total elongation TE of the cold rolledsteel sheet is of at least 19%, the composition of the steel is suchthat 0%<Cr≦0.5%, 0%<Mo≦0.3%, and the partitioning time is between 15seconds and 150 seconds. Preferably, according to this embodiment, thereis no addition of Nb.

Thus, according to this embodiment, the invention relates to a methodfor producing a cold rolled steel sheet having a tensile strength TS ofat least 1470 MPa and a total elongation TE of at least 19%, the methodcomprising the successive steps of:

-   -   annealing at an annealing temperature AT a cold rolled steel        sheet made of steel whose chemical composition contains in        weight %:

0.34%≦C≦0.40%

1.50%≦Mn≦2.30%

1.50≦Si≦2.40%

0%<Cr≦0.5%

0%<Mo≦0.3%

0.01%≦Al≦0.08%

the remainder being Fe and unavoidable impurities, the annealingtemperature AT being equal or higher than the Ac3 transformation pointof the steel, to obtain an annealed steel sheet,

-   -   quenching the annealed steel sheet by cooling it down to a        quenching temperature QT lower than the Ms transformation point        of the steel, typically between 150° C. and 250° C., to obtain a        quenched steel sheet, and,    -   making a partitioning treatment by reheating the quenched steel        sheet at a partitioning temperature PT between 350° C. and        450° C. and maintaining the steel sheet at this temperature        during a partitioning time Pt between 15 seconds and 150        seconds.

In two embodiments, after partitioning the steel sheet is cooled to theroom temperature in order to obtain a non-coated steel sheet:

In the first of these two embodiments, the composition of the steel issuch that 0.36%≦C≦0.40%, Cr<0.05% and Mo<0.05%, the quenchingtemperature is between 190° C. and 210° C. and the partitioning time Ptis between 90 seconds and 110 seconds.

In the second of these two embodiments, the composition of the steel issuch that 0.34%≦C≦0.37%, 0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%, thequenching temperature is between 200° C. and 230° C. and thepartitioning time Pt is between 25 seconds and 120 seconds.

Preferably, the bare cold rolled steel is afterwards electro-galvanized.

In another embodiment, after partitioning the steel sheet is galvanizedthen cooled to the room temperature in order to obtain a coated steelsheet, the composition of the steel is such that 0.34%≦C≦0.37%,0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%, the quenching temperature is between200° C. and 230° C. and the partitioning time Pt is between 25 secondsand 55 seconds.

Thus, in a preferred embodiment, the composition of the steel is suchthat, 0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%, and preferably such that0.34%≦C≦0.37%.

With this preferred embodiment, if after partitioning the steel sheet iscooled to the room temperature in order to obtain a non-coated steelsheet, the quenching temperature is preferably between 200° C. and 230°C. and the partitioning time Pt is preferably between 15 seconds and 120seconds.

Still with this preferred embodiment, if after partitioning the steelsheet is galvanized then cooled to the room temperature in order toobtain a coated steel sheet, the quenching temperature is preferablybetween 200° C. and 230° C. and the partitioning time Pt is preferablybetween 25 seconds and 55 seconds.

According to another embodiment, the composition of the steel is suchthat 0.46%≦Cr≦0.7% and/or 0.03%≦Nb≦0.05%, and preferably such that0%≦Mo≦0.005%.

According to this embodiment, after partitioning the steel sheet ispreferably coated then cooled to the room temperature in order to obtaina coated steel sheet.

According to this embodiment, the coating step is for example agalvanizing step. Preferably, the quenching temperature is between 200°C. and 230° C. and the partitioning time Pt is between 50 seconds and250 seconds.

The coating step may be a galvannealing step with an alloyingtemperature GA between 470 and 520° C., preferably between 480° C. and500° C., the sheet being maintained at the alloying temperature GA for atime comprised between 5 s and 15 s. Preferably, the quenchingtemperature is then between 200° C. and 230° C. and the partitioningtime Pt between 40 s and 120 s.

The invention relates also to a coated or non-coated steel sheet made ofsteel whose chemical composition comprises in weight %:

0.34%≦C≦0.40%

1.50%≦Mn≦2.30%

1.50≦Si≦2.40%

0%<Cr≦0.7%

0%≦Mo≦0.3%

0.01%≦Al≦0.08%

and optionally 0%≦Nb≦0.05%,

the remainder being Fe and unavoidable impurities, the structurecomprising at least 60% of martensite and between 12% and 15% ofresidual austenite, the tensile strength is at least 1470 MPa and thetotal elongation being at least 16%.

In a particular embodiment, the steel is such that 0%<Cr≦0.5% and0%<Mo≦0.3%.

The total elongation of the sheet is preferably at least 19%.

Thus, the invention relates in particular to a coated or non-coatedsteel sheet made of steel whose chemical composition comprises in weight%:

0.34%≦C≦0.40%

1.50%≦Mn≦2.30%

1.50≦Si≦2.40%

0%<Cr≦0.5%

0%≦Mo≦0.3%

0.01%≦Al≦0.08%

the remainder being Fe and unavoidable impurities, the structurecomprising at least 60% of martensite and between 12% and 15% ofresidual austenite, the tensile strength is at least 1470 MPa and thetotal elongation being at least 19%.

In a particular embodiment, the steel sheet is non-coated, thecomposition of the steel is such that 0<Cr<0.05% and 0<Mo<0.05%, and theyield strength is higher than 1150 MPa. Preferably, there is no additionof Nb.

In another embodiment, the steel sheet is non-coated, the composition ofthe steel is such that 0.35≦Cr≦0.45% and 0.07≦Mo≦0.20%, and the yieldstrength is higher than 880 MPa, the tensile strength is higher than1520 MPa, and the total elongation is of at least 20%. Preferably, thereis no addition of Nb.

In another embodiment, the steel sheet is galvanized, the composition ofthe steel is such that 0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%, the tensilestrength is higher than 1510 MPa and the total elongation is at least20%. Preferably, there is no addition of Nb.

Thus, according to a preferred embodiment, the composition of the steelis such that 0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%. If the sheet is notcoated, the yield strength may be higher than 880 MPa, the tensilestrength higher than 1520 MPa and the total elongation of at least 20%.If the sheet is galvanized, the tensile strength may be higher than 1510MPa and the total elongation of at least 20%.

According to another preferred embodiment, the composition of the steelis such that 0.46%≦Cr≦0.7%, and/or 0.03%≦Nb≦0.05%. Preferably, thecomposition of the steel is such that 0%≦Mo≦0.005%.

Preferably, with this preferred embodiment, at least one face of thesheet is galvanized or galvannealed.

The invention will now be described in details but without introducinglimitations.

According to the invention, the sheet is obtained by heat treating a hotor preferably a cold rolled non-treated steel sheet made of steel whichchemical composition contains, in weight %:

-   -   0.34% to 0.40% of carbon to ensure a satisfactory strength and        improve the stability of the retained austenite. This is        necessary to obtain a sufficient elongation. If carbon content        is too high, the hot rolled sheet is too hard to cold roll and        the weldability is insufficient.    -   1.50% to 2.40% of silicon in order to stabilize the austenite,        to provide a solid solution strengthening and to delay the        formation of carbides during partitioning with appropriate        procedures to prevent the formation of silicon oxides at the        surface of the sheet which is detrimental to coatability.    -   1.50% to 2.30% of manganese to have a sufficient hardenability        in order to obtain a structure containing at least 60% of        martensite, a tensile strength of more than 1470 MPa and to        avoid having segregation issues which are detrimental for the        ductility.    -   0% to 0.3% of molybdenum and 0% to 0.7% of chromium to increase        the hardenability and to stabilize the retained austenite in        order to strongly reduce austenite decomposition during        partitioning. The absolute zero value is excluded due to        residual amounts. According to an embodiment, the composition        comprises from 0% to 0.5% of chromium. When the steel sheet is        non-coated, the molybdenum and the chromium can be eliminated        and their contents can remain less than 0.05% each. When the        steel sheet is coated by galvanizing, the molybdenum content is        preferably from 0.07% to 0.20% and the chromium content is        preferably from 0.35% to 0.45%. As an alternative, when the        sheet is coated, in particular by galvannealing, the molybdenum        content is preferably lower than 0.005%, and the chromium        content is preferably from 0.46% to 0.7%. A molybdenum content        lower than 0.005% corresponds to the presence of molybdenum only        as an impurity or a residual.    -   0.01% to 0.08% of aluminum which is usually added to liquid        steel for the purpose of deoxidation, preferably.

The remainder is iron and residual elements or unavoidable impuritiesresulting from the steelmaking. In this respect, Ni, Cu, V, Ti, B, S, Pand N at least are considered as residual elements which are unavoidableimpurities. Therefore, generally, their contents are less than 0.05% forNi, 0.05 for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for Pand 0.010% for N.

Addition of microalloy elements such as Nb from 0 to 0.05% and/or Tifrom 0 to 0.1% could be utilized to obtain the desired microstructureand an optimal combination of product properties.

In particular, when the sheet is coated, Nb may be added in an amount upto 0.05%. According to an embodiment, Nb is preferably comprised between0.03 and 0.05%. According to this embodiment, the sheet is preferablycoated, by galvanizing or galvannealing. A Nb content of 0.03 to 0.05%allows obtaining satisfactory tensile strength and elongation, inparticular a tensile strength of at least 1470 MPa and an elongation ofat least 16%, when the sheet is coated by galvanizing or galvannealing.

Thus, when the sheet is coated, in particular by galvannealing, thecomposition may comprise Nb in an amount between 0.03% and 0.05%, Cr inan amount between 0.46% and 0.7%, and no addition of Mo.

The non-treated steel sheet is a cold rolled sheet prepared according tothe methods known by those who are skilled in the art.

After rolling the sheets are pickled or cleaned then heat treated andoptionally hot dip coated.

The heat treatment which is made preferably on a continuous annealingwhen the sheet is not coated and on a hot dip coating line when thesteel sheet is coated, comprises the following successive steps:

-   -   annealing the cold rolled sheet at an annealing temperature AT        equal or higher than the Ac3 transformation point of the steel,        and preferably higher than Ac3+15° C., in order to obtain an        annealed steel sheet having a structure completely austenitic,        but less than 1000° C. in order not to coarsen too much the        austenitic grains. Generally, a temperature higher than 870° C.        is enough for the steel according to the invention and this        temperature does not need to be higher to 930° C. Then the steel        sheet is maintained at this temperature i.e. maintained between        AT−5° C. and AT+10° C., for a time sufficient to homogenize the        temperature in the steel. Preferably, this time is of more than        30 seconds but does not need to be more than 300 seconds. To be        heated to the annealing temperature, the cold rolled steel sheet        is, for example, first heated to a temperature of about 600° C.        at a speed typically below 20° C./s then heated again to a        temperature of about 800° C. at a speed typically below 10° C./s        and eventually heated to the annealing temperature at a heating        speed below 5° C./s. In this case, the sheet is maintained at        the annealing temperature for a duration between 40 and 150        seconds.    -   quenching of the annealed sheet by cooling down to a quenching        temperature QT lower than the Ms transformation point between        150° C. and 250° C. at a cooling rate fast enough to avoid        ferrite formation upon cooling and preferably of more than 35°        C./second, in order to obtain a quenched sheet having a        structure consisting of martensite and austenite, then the final        structure contains at least 60% of martensite and between 12%        and 15% of austenite. If the steel contains less than 0.05% of        molybdenum and less than 0.05% of chromium, the quenching        temperature is preferably between 190° C. and 210° C. When the        steel sheet has to be galvanized and when the chemical        composition of the steel is such that 0.34%≦C≦0.37%,        0.35%≦Cr≦0.45% and 0.07%≦Mo≦0.20%, then the quenching        temperature is preferably between 200° C. and 230° C. When the        composition of the steel is such that 0.46%≦Cr≦0.7% and        0%≦Mo≦0.005%, the quenching temperature is also preferably        between 200° C. and 230° C.    -   reheating the quenched sheet up to a partitioning temperature PT        between 350° C. and 450° C. The heating speed is preferably at        least 30° C./s.    -   maintaining the sheet at the partitioning temperature PT for a        partitioning time Pt between 15 sec and 250 sec, for example        between 15 sec and 150 sec. During the partitioning step, the        carbon is partitioned, i.e. diffuses from the martensite into        the austenite which is thus enriched.    -   Optionally, cooling the sheet down to the room temperature if no        coating is desired or heating the sheet to a coating        temperature, hot dip coating the sheet and cooling it down to        the room temperature if a coating is desired. The hot dip        coating is, for example, galvanizing, and the coating        temperature is about 460° C. as it is known in the art.

The heating to the coating temperature is made preferably at a heatingspeed of at least 30°/s and the coating takes between 2 and 10 s.

According to a particular embodiment, the hot dip coating isgalvannealing. In this embodiment, the partitioning time is preferablycomprised between 40 s and 120 s, for example higher than or equal to 50s and/or lower than or equal to 100 s.

The sheet is heated from the partitioning temperature PT to the coatingtemperature, which is in this case an alloying temperature, and cooleddown to room temperature after galvannealing.

The heating to the alloying temperature is made preferably at a heatingspeed of at least 20° C./s, preferably at least 30° C./s.

Preferably, the alloying temperature is lower than 520° C. and higherthan 470° C. Still preferably, the alloying temperature is lower than orequal to 500° C. and/or higher than or equal to 480° C.

The sheet is maintained at the alloying temperature for a time which isfor example comprised between 5 s and 20 s, preferably between 5 s and15 s, for example between 8 s and 12 s. Indeed, maintaining the sheet atthe alloying temperature for more than 20 s, leads to a reduction of theductility, in particular to a decrease in the total elongation of thesheet.

Whether or not a coating is applied, the cooling speed to the roomtemperature is preferably between 3 and 20° C./s.

When the sheet is not coated and the steel contains preferably less than0.05% of chromium and less than 0.05% of molybdenum, then thepartitioning time is preferably between 90 sec and 110 sec. With suchtreatment it is possible to obtain sheets having a yield strength ofmore than 1150 MPa, a tensile strength of more than 1470 MPa and a totalelongation of more than 19%.

When the sheet is not coated and the steel contains 0.35% and 0.45% ofchromium and between 0.07% and 0.20% of molybdenum, then thepartitioning time is preferably between 15 sec and 120 sec. With suchtreatment it is possible to obtain sheets having a yield strength ofmore than 880 MPa, a tensile strength of more than 1520 MPa and a totalelongation of more than 20%.

When the sheet is coated, the composition and the treatment parametersare preferably adjusted according to the two following embodiments.

According to a first embodiment, when the sheet is coated, the steelcontains preferably between 0.35% and 0.45% of chromium and between0.07% and 0.20% of molybdenum and the partitioning time Pt is preferablybetween 25 seconds and 55 seconds. In these conditions it is evenpossible to obtain coated steel sheet having a tensile strength higherthan 1510 MPa and a total elongation of at least 20%.

According to a second embodiment, when the sheet is coated, the steelmay comprise between 0.46 and 0.7% of Cr, less than 0.005% of Mo andbetween 0.03 and 0.05% of Nb. With this composition, the partitioningtime is preferably higher than 30 s, still preferably higher than orequal to 50 s.

When the sheet is coated by galvanizing, the partitioning time may be ashigh as 230 s.

When the sheet is coated by galvannealing, the partitioning time Pt ispreferably between 40 seconds and 120 seconds, still preferably between50 and 100 seconds. The alloying temperature is preferably comprisedbetween 470° C. and 520° C., still preferably between 480° C. and 500°C.

The sheet is preferably maintained at the alloying temperature for lessthan 20 s, preferably less than 15 s, and more than 5 s. In theseconditions it is possible to obtain a galvannealed steel sheet having atensile strength higher than 1470 MPa, even higher than 1510 MPa, and atotal elongation of at least 16%.

As examples and comparison, it was manufactured sheets made of steelswhose compositions in weight and characteristic temperatures such as Ac3and Ms are reported in table I.

The sheets were cold rolled, annealed, quenched, partitioned and cooledto the room temperature or, galvanized after partitioning before beingcooled to the room temperature.

The mechanical properties were measured in the transverse directionrelative to the direction of rolling. As it is well known in the art,the ductility level is slightly better in the direction of rolling thanin the transverse direction for such high strength steel. Measuredproperties are Hole expansion ratio HER measured according to thestandard ISO 16630:2009, the yield strength YS, the tensile stress TS,the uniform elongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties are reportedin Table II for the non coated sheets and in Table III for the coatedsheets.

In these tables, AT is the annealing temperature, QT the quenchingtemperature, PT the partitioning temperature. In Table II, GI is thetemperature of galvanizing.

TABLE I Ref C Mn Si Cr Mo Al Ac3 Ms steel % % % % % % ° C. ° C. S1800.29 2.02 2.44 0.004 Residual 0.059 920 290 (<0.003) S181 0.39 2.03 1.950.003 Residual 0.058 860 240 (<0.003) S80 0.36 1.99 1.95 0.41 0.0880.045 850 250 S81 0.38 1.98 1.93 0.34 0.14 1.047 860 270

TABLE II Ex- AT QT PT Pt HE YS TS UE TE ample Steel ° C. ° C. ° C. sec %MPa MPa % % 1 S180 920 240 400 10 — 982 1497 11.4 15.9 2 S180 920 240400 100 17 1073 1354 13.9 19.9 3 S180 920 240 400 500 — 1082 1309 13.218.4 4 S181 900 200 400 10 — 1095 1583 12.5 13.8 5 S181 900 200 400 10021 1238 1493 13.0 19.4 6 S181 900 200 400 500 — 1207 1417 13.1 17.7 7S80 900 220 400 10 — 925 1518 6.6 6.8 8 S80 900 220 400 30 — 929 14388.9 8.9 9 S80 900 220 400 50 — 897 1462 13.5 18.5 10 S80 900 220 400 100— 948 1447 15.7 19.6 11 S81 900 240 400 10 — 867 1623 8.1 9.3 12 S81 900240 400 30 — 878 1584 11.4 11.8 13 S81 900 240 400 50 — 833 1520 10.812.2 14 S81 900 240 400 100 — 840 1495 15.9 17.3

TABLE III Ex- AT QT PT GI Pt HE YS TS UE TE ample Steel ° C. ° C. ° C. °C. sec % MPa MPa % % 15 S180 920 240 400 460 100 24 1127 1310 13.7 20.716 S181 900 200 400 460 10 — 933.4 1348 14.0 18.0 17 S181 900 200 400460 30 — 1170 1425 13.8 20.1 18 S181 900 180 400 460 100 — 1353 1507 8.014.1 19 S181 900 200 400 460 100 19 1202 1399 13.0 20.2 20 S181 900 220400 460 100 — 936 1280 14.3 18.0 21 S181 900 200 420 460 10 — 906 134611.2 10.6 22 S181 900 200 420 460 30 — 841 1298 14.7 19.3 23 S181 900200 420 460 100 — 900 1322 14.5 19.1 24 S181 900 200 360 460 10 — 9101357 14.5 19.0 25 S181 900 200 360 460 30 — 992 1356 14.0 18.9 26 S80900 220 400 460 10 — 756 1576 10.5 11.1 27 S80 900 220 400 460 30 — 8361543 18.3 20.3 28 S80 900 220 400 460 50 — 906 1534 18.6 21.6 29 S80 900220 400 460 100 — 941 1394 8.1 8.58 30 S81 900 240 400 460 10 — 925 16589.4 9.39 31 S81 900 240 400 460 30 — 929 1603 15.1 20.5 32 S81 900 240400 460 50 — 897 1554 16.1 21.1 33 S81 900 240 400 460 100 — 948 154218.1 21.4

The examples 1 to 14 show that it is only with the steel S181, whichcontains neither chromium nor molybdenum, and steel S80, which containsboth chromium and molybdenum, that it is possible to reach the desiredproperties i.e. TS≧1470 MPa and TE≧19%. In alloy S181, the desiredproperties are achieved for a quenching temperature QT of 200° C. and apartitioning time of 100 seconds. In this case, the yield strength ishigher than 1150 MPa. In alloy S80, which contains chromium andmolybdenum, the desired properties are achieved for a quenchingtemperature QT of 220° C. and a partitioning time between 30 to 100seconds (examples 7 to 10). In this case, the tensile strength is higherthan 1520 MPa and the total elongation is more than 20%. Moreover, it isworth mentioning that all the examples containing Cr and Mo (7 to 14)have yield strengths significantly lower than the examples 1 to 6,concerning a steel without Cr and Mo.

The examples 15 to 33 show that only the examples corresponding tosteels containing Cr and Mo are able to reach the desired propertieswhen the sheets are galvanized (examples 27 and 28). For the steel S80,the quenching temperature has to be of 220° C. and a partitioning of 10seconds is too short while a partitioning time of 100 seconds is toolong. When the steel does not contain Cr and does not contain Mo, thetensile strength always remains lower than 1470 MPa.

Other sheets made of an alloy having the composition shown in Table IVwere cold rolled, annealed, quenched, partitioned, galvanized orgalvannealed and cooled to the room temperature.

TABLE IV C Mn Si Cr Mo Al Ac3 Ms % % % % % % Nb ° C. ° C. 0.38 1.98 1.930.51 0.003 0.048 0.039 825 290

The mechanical properties of the sheets were measured in the transversedirection relative to the direction of rolling. As it is well known inthe art, the ductility level is slightly better in the direction ofrolling than in the transverse direction for such high strength steel.Measured properties are the Hole Expansion Ratio HER measured accordingto the standard ISO 16630:2009, the yield strength YS, the tensilestrength TS, the uniform elongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties of thegalvanized sheets are reported in Table V.

In this table, GI is the temperature of galvanizing.

TABLE V AT QT PT Pt GI HE YS TS UE TE Example ° C. ° C. ° C. sec ° C. %MPa MPa % % 34 900 205 400 30 460 — 1032 1624 14 15.7 35 900 205 400 50460 — 1102 1606 16.1 19.8 36 900 205 400 150 460 — 1139 1594 15.3 20.937 900 205 400 230 460 — 1179 1606 15.2 19.2

Examples 35 to 37 show that with a steel comprising higher amounts ofchromium and niobium and a lower amount of molybdenum, the desiredproperties, i.e. TS≧1470 MPa and TE≧19%, can be reached with apartitioning time of more than 30 s, in particular of at least 50 s.

The conditions of treatment and the mechanical properties of thegalvannealed sheets are reported in Table VI.

In this table, TGA is the alloying temperature and t_(GA) is the holdingtime at this alloying temperature TGA.

AT QT PT Pt TGA t_(GA) HE YS TS UE TE Example ° C. ° C. ° C. sec ° C.sec % MPa MPa % % 38 900 205 400 30 500 20 — 850 1589 9.8 12.8 39 900205 400 50 500 20 — 858 1563 12.1 12 40 900 205 400 100 500 20 — 8811534 13.4 15.7 41 900 205 400 50 500 10 — 1062 1548 14.7 16.5 42 900 205400 100 500 10 — 990 1561 14.3 16.5 43 900 205 400 150 500 10 — 998 158112.7 14.3 44 900 205 400 50 480 10 — 1035 1603 14.4 17.9

Examples 38-44 show that a partitioning time Pt between 40 seconds and120 seconds, in particular between 50 and 100 seconds, allow obtaining agalvannealed steel sheet having a tensile strength higher than 1510 MPaand a total elongation of at least 16%.

In particular, example 44 show that an alloying temperature of 480° C.and a holding time at the alloying temperature of 10s even allowobtaining a tensile strength of more than 1510 MPa and a totalelongation of more than 16%, even more than 17%.

1-31. (canceled)
 32. A method for producing a coated steel sheet havinga tensile strength TS of at least 1470 MPa and a total elongation TE ofat least 16%, the method comprising the steps of: annealing at anannealing temperature AT a cold rolled steel sheet made of a steelhaving a chemical composition containing in weight %:0.34%≦C≦0.40%;1.50%≦Mn≦2.30%;1.50≦Si≦2.40%;0.35%≦Cr≦0.45%;0.07%≦Mo≦0.20%;0.01%≦Al≦0.08%;and0%≦Nb≦0.05%; a remainder being Fe and unavoidable impurities, theannealing temperature AT being higher than an Ac3 transformation pointof the steel, to obtain an annealed steel sheet; quenching the annealedsteel sheet by cooling the steel sheet down to a quenching temperatureQT lower than an Ms transformation point of the steel and comprisedbetween 200° C. and 230° C., to obtain a quenched steel sheet;performing a partitioning treatment by reheating the quenched steelsheet at a partitioning temperature PT between 350° C. and 450° C. andmaintaining the quenched steel sheet at the partitioning temperature PTduring a partitioning time Pt between 25 seconds and 55 seconds;galvanizing the steel sheet; then cooling the steel sheet to roomtemperature in order to obtain a coated steel sheet.
 33. The methodaccording to claim 32, wherein during quenching, the annealed steelsheet is cooled down to said quenching temperature at a cooling ratefast enough to avoid ferrite formation upon cooling, in order to obtaina structure of the quenched steel sheet consisting of martensite andaustenite.
 34. The method according to claim 32, wherein the annealingtemperature AT is between 870° C. and 930° C.
 35. The method accordingto claim 32, wherein the chemical composition of the steel includes0.34%≦C≦0.37%.
 36. A method for producing a steel sheet having a tensilestrength TS of at least 1470 MPa and a total elongation TE of at least16%, the method comprising the steps of: annealing at an annealingtemperature AT a cold rolled steel sheet made of a steel having achemical composition containing in weight %:0.34%≦C≦0.40%;1.50%≦Mn≦2.30%;1.50≦Si≦2.40%;0.46%≦Cr≦0.07%;0%≦Mo≦0.3%;0.01%≦Al≦0.08%;and0.03%≦Nb≦0.05%; a remainder being Fe and unavoidable impurities, theannealing temperature AT being higher than an Ac3 transformation pointof the steel, to obtain an annealed steel sheet; quenching the annealedsteel sheet by cooling the steel sheet down to a quenching temperatureQT lower than an Ms transformation point of the steel and between 150°C. and 250° C., to obtain a quenched steel sheet; and performing apartitioning treatment by reheating the quenched steel sheet to apartitioning temperature PT between 350° C. and 450° C. and maintainingthe quenched steel sheet at the partitioning temperature PT during apartitioning time Pt between 15 seconds and 250 seconds.
 37. The methodaccording to claim 36, wherein, during quenching, the annealed steelsheet is cooled down to said quenching temperature at a cooling ratefast enough to avoid ferrite formation upon cooling, in order to obtaina structure of the quenched steel sheet consisting of martensite andaustenite.
 38. The method according to claim 36, wherein the annealingtemperature AT is between 870° C. and 930° C.
 39. The method accordingto claim 36, wherein the chemical composition of the steel includes0%≦Mo≦0.005%.
 40. The method according to claim 36, wherein, afterpartitioning, the steel sheet is coated then cooled to room temperaturein order to obtain a coated steel sheet.
 41. The method according toclaim 40, wherein the coating step is a galvanizing step.
 42. The methodaccording to claim 41, wherein the quenching temperature is between 200°C. and 230° C. and the partitioning time Pt is between 50 seconds and250 seconds.
 43. The method according to claim 40, wherein the coatingstep is a galvannealing step with an alloying temperature GA between470° C. and 520° C., and the steel sheet is maintained at the alloyingtemperature GA for a time between 5 s and 15 s.
 44. The method accordingto claim 43, wherein the alloying temperature GA is between 480° C. and500° C.
 45. The method according to claim 43, wherein the quenchingtemperature is between 200° C. and 230° C. and the partitioning time Ptis between 40 s and 120 s.
 46. A coated steel sheet comprising: a steelhaving a chemical composition comprising in weight %:0.34%≦C≦0.40%;1.50%≦Mn≦2.30%;1.50≦Si≦2.40%;0.35%≦Cr≦0.45%;0.07%≦Mo≦0.20%;0.01%≦Al≦0.08%;and0%≦Nb≦0.05%; a remainder being Fe and unavoidable impurities; and astructure comprising at least 60% of martensite and between 12% and 15%of residual austenite; the coated steel sheet being galvanized, thecoated steel sheet having a tensile strength of at least of 1510 MPa anda total elongation of at least 20%.
 47. A coated or non-coated steelsheet comprising: a steel having a chemical composition comprising inweight %:0.34%≦C≦0.40%;1.50%≦Mn≦2.30%;1.50≦Si≦2.40%;0.46%≦Cr≦0.7%;0%≦Mo≦0.3%;0.01%≦Al≦0.08%;and0.03%≦Nb≦0.05%; a remainder being Fe and unavoidable impurities; astructure comprising at least 60% of martensite and between 12% and 15%of residual austenite; the steel sheet having a tensile strength of atleast of 1470 MPa and a total elongation being at least 16%.
 48. Thesteel sheet according to claim 47, wherein the chemical composition ofthe steel includes 0%≦Mo≦0.005%.
 49. The steel sheet according to claim48, wherein at least one face of the steel sheet is galvanized.
 50. Thesteel sheet according to claim 48, wherein at least one face of thesteel sheet is galvannealed.